Thermal control interface coatings and pigments

ABSTRACT

The invention provides an optical structure with low chroma and brightness in the visible region and low emissivity in the infrared region. The optical structure includes an interference structure having an infrared reflective layer and an infrared absorbing thin film layer. These layers are in turn separated by a thin film spacer of a dielectric or semiconductor material. The reflectivity and transmission of the layers are selectively controlled through the thickness of the layers such that the visual reflectivity and color is independent of the infrared properties of the absorber and reflector layers.

CROSS REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable

REFERENCE TO SEQUENCE LISTING

Not applicable

BACKGROUND OF THE INVENTION

This invention relates to partially reflective coatings useful forcontrolling thermal radiation, that is infrared reflective coatings thatthat have selectively controlled optical properties in visualwavelengths permitting a wide variation in perceived color andbrightness having optimal infrared optical properties.

Various methods have been used to achieve thermal radiation control ofobjects by selectively controlling the object's reflectivity to infraredradiation. Methods generally involve either applying a coating to anobject or forming its outer surface of a material having high infraredreflectivity. Thermal radiation controlled objects and surfaces have anumber of uses, among which are solar collector absorber panels, spacevehicle surfaces, and camouflaging military vehicles from detection byinfrared scanning. In many cases, it is desirable to selectively reflectspecific wavelengths of the infrared radiation while attenuating othersby absorption. For example, for solar energy collection it is desirablethat the surface coating absorb radiation corresponding to the sun'ssolar emission spectrum, that is principally from 300 to 2,500nanometers, while having a higher reflectivity at longer (generallyabove about 4 microns) “thermal” wavelengths. This allows the object toabsorb and retain the sun's heat because the increased reflectivity atthe thermal wavelengths decreases emittance of these wavelengths. Thetemperature of the object generally determines the wavelength of thethermal emissions. For an object in thermal equilibrium with itssurroundings having a given temperature, the term emmitance is definedat the ratio of the energy emitted by such object divided by the energythat would be emitted by a perfect black body at the same temperature.For an object at room temperature (27 degrees Celsius, 300 degreesKelvin), emittance can be written as:$ɛ_{300K} = \frac{\int_{4}^{40}{\left( {1 - {R(\lambda)} - {T(\lambda)}} \right){{BB}\left( {\lambda,{300K}} \right)}\quad{\mathbb{d}\lambda}}}{\int_{4}^{40}{{{BB}\left( {\lambda,{300K}} \right)}\quad{\mathbb{d}\lambda}}}$where R(λ) and T(λ) are the reflectance and transmittance of the objectat each wavelength, respectively, BB(λ, 300 K) is shorthand for theso-called black-body function predicting the amount of energy emitted bya 300° K. perfect black body at each wavelength, and the limits ofintegration are 4 micrometers to 40 micrometers. For an opaque object,the transmittance is zero and hence the higher the IR reflectance of theobject, the lower the IR emittance. Conversely, for opaque objects, thelower the IR reflectance of the object, the higher the IR emittance.

Differential absorption and reflectivity of solar and thermalwavelengths, can be achieved by first metallizing a surface with highlypolished or reflective metal foils or coatings, as many metals arehighly reflective in the solar and far-infrared (thermal) regions of thespectrum. Selective absorption in the solar wavelengths can be achievedby over coating the reflective metallic surface with materials thatselectively absorb solar wavelengths. The most common methods forforming such surfaces are by electrochemical deposition techniquesfollowed by chemical oxidation of the deposit and by “paint” technologyusing organic based coatings.

In the former case, a suitable substrate, such as aluminum, iselectroplated with copper. The copper surface is then chemicallyoxidized to form a surface layer of cupric oxide. One objection to thismethod is the high cost of the combined electrochemical/chemicaloxidation process to obtain the desired surface. Another disadvantage isthat one cannot select the visible color or appearance of the compositestructure, which is the color of cupric oxide.

Paint technology has been used to give objects selective radiationproperties. Some paints use organic solvents and organic binders with anadditive. A particular example is a lead sulfide/silicone resin binderin xylene. However, organic-based paints typically release volatileorganic compounds, which may be controlled or even prohibited in someareas because of environmental concerns. Further, such organic paintsmay not provide sufficient radiation control properties for someapplications.

Water-based paints have also been investigated. Black silicate paint hasbeen developed that uses a suitable pigment bound in an alkali metalsilicate, such as sodium silicate. Such a formulation is sprayable andachieves an effect similar to that of the electrochemical process or byorganic binder paint technology. Unfortunately, the pigments can reactchemically with the silicate binders in some instances to form pigmentsilicate salts or complexes. These compounds alter the properties ofabsorptance and emittance of the coating to such an extent that theperformance of the coating may degrade to an unacceptable level.

Another approach is to use two water-based coatings in which a layer ofa semiconductor pigment is first deposited upon a thermally reflectivesubstrate and then this pigment layer is overcoated with an alkali metalsilicate binder. The silicate layer is heat cured at above ambienttemperatures to form a protective coating over the pigment.

The technology of thermal radiation control surfaces is based on theneed to obtain a surface that absorbs radiation in the range of 300 to2,500 nanometers while at the same time suppressing emission of thermalenergy. This basic principle accounts for the operation of solarcollector absorber panels, infrared transparent coatings used onmilitary equipment and the like. The general approach is to start with asubstrate material that has high reflectance (low absorbance) over theentire spectral range including the incident radiation and potentialemission (300 to 40,000 nanometers, for example). Examples of suchuseful substrates include metals such as aluminum, copper, steel and thelike, and non-metallic substrates, such as plastics and glass, which canbe metallized to provide a highly reflective surface.

In order to obtain the desired properties of opaqueness to ultravioletand visible light and transparency to infrared radiation, it isdesirable to form a coating on the highly reflective substrate thatabsorbs in the visible and ultraviolet region while transmittinginfrared. The combination of the coating on the substrate is preferablyhighly absorbing in the visible range and highly reflecting (lowemitting) in the thermal range. This makes semiconductor pigments highlydesirable, as these compounds are highly transparent in the infrared,but absorb in the visible region. Not all semiconductor pigments areuseful, as those having a high refractive index and thus a high surfacereflection coefficient give rise to unacceptable reflection losses.Thus, only those semiconductor pigments having low enough refractiveindices to keep surface reflectivity at a minimum are acceptable. Amongsuch useful semiconductor pigments are copper oxide, iron oxides, bothnaturally-occurring and synthetically made, chromium oxides, nickeloxide, complexes of nickel-zinc-sulfide, lead sulfide and so forth.Since thermal and photochemical stability is required of thesemiconductor, organic dyes would not be very useful and the preferredsemiconductors are, therefore, the inorganic pigments alreadyenumerated.

Such thermal control surfaces have undesirable visual appearances formany applications because the broadband reflectors are very bright ormetallic in appearance, while selective absorbers have a blackappearance in the visible.

One approach uses a paint composition to achieve a diffused visualblue-gray coating of non-metallic texture for use on metal surfaces thatprovides reduced infrared emittance. Previous camouflage coatings andpaints used on hulls of naval vessels often exhibited relatively highsolar absorption because of the dark colors and diffused finishes thatare characteristic of the coatings. This high solar absorption resultedin high surface temperatures that increase cooling requirements and moreimportantly increased infrared radiation. In modern warfare, infrareddetection techniques have become highly developed and means forcounter-detection techniques are accordingly required. Artificialcooling of hot exposed surfaces is effective to reduce infraredemission. However, this method increases electrical power requirementsaboard ship as well as adding parasitic weight and volume to equipmentaboard the ship.

This low infrared emittance coating is applied as a paint to provide adurable opaque coating suitable for use on exposed surfaces of navalvessels or on hot surfaces of a gas turbine exhaust. Ideally, suchcoated surfaces exhibit low reflectance in the visual portion of thelight wavelengths and high reflectance in the infrared portion. Thepaint is a mixture of colorant and emitance control pigments such asaluminum, zinc sulfide, antimony trisulfide, and blue pigments; aluminumoxide filler; silicon alkyd resin binder; polarized montmorillite clay;and a diluent. Like traditional military paints, it utilizes somefraction of visually absorptive pigments, that do not have wavelengthspecific or optimized infrared properties; their reflectance tends to beconstant over different wavelength bands, which compromises its infraredperformance.

Various flakes or pigments have been made that have optically selectiveor optically variable properties. Some optically selective pigments haveinterference structures that enhance or suppress a portion of thevisible spectrum to achieve a desired color, and are generally used incolorful paints, inks, plastics, and other carriers. Some opticallyvariable pigments are similarly directed at the visible spectrum andshift color as the location of the observer changes. The interferencestructures typically include thin film layers of spacer (dielectric) andabsorber materials over a reflector. Similar flakes utilize opticalcoating structures to selectively absorb solar radiation in paintintended for passive solar energy systems; however, the thermalemittance characteristics of the paint appears to be influenced by theinfrared absorption spectra of the paint vehicle.

Prior technology for thermal control of visually opaque objects resultedin either a highly reflective metallic appearance, in the case ofbroadband visual IR reflectors, or a black color, in that materialsselectively absorbing in the solar IR region are also absorbing invisual wavelengths. Other attempts at reducing the chroma of coatingsoften include adding a darkening agent to the carrier or binder. Thisoften detracts from the IR performance of the coating. Attempts toachieve other visual colors resulted in some compromise of the thermalcontrol properties

Accordingly, it would be desirable to provide flake-like pigments thathave low reflection in visible light in a range of colors, withselectable visible color characteristics being independent from their IRcharacteristics.

Another object of the present invention is to provide thermal control ofvisually opaque objects, especially those having arbitrary or irregularshape by application of a coating or foil that allows selectiveabsorption of light at visible wavelengths, and reflection of light atIR wavelengths. It is further desirable that thermal control coatingshave a range of independently selected color and chroma in the visiblewavelengths.

Another objective of the present invention to provide efficient solarenergy collection absorbing material with low thermal emittance, andthat solar energy collection absorbing material be available in aselection of colors.

BRIEF SUMMARY OF THE INVENTION

The present invention provides optical interference structures thatappear dark or have low chroma in the visible portion of the spectrumand relatively high reflectivity in the infrared portion of thespectrum. In one embodiment the optical interference structure includesa reflector, spacer layer, and absorber layer. The absorber layer has athickness that provides a transmittance of between about 5-85%. In otherembodiments the thickness of the absorber layer and spacer layer areselected according to the reflectivity of the reflective layer. In someembodiments, the optical interference structures have a chroma of lessthan 50 and a reflectivity in the infra-red portion of the spectrum ofmore than 50%, and in some cases greater than 80%.

In some embodiments the optical interference structures are used inpigments, such as by forming the optical interference structure on oneor both sides of a pigment flake, or forming the structures on a roll ofpolymer film, and then separating and processing the deposited filmstructure into pigment flakes. In other embodiments, the opticalinterference structures are deposited onto a substrate, such as a sheetof foil, that is than attached to an object, or directly onto an object,such as a panel. The foil or object can be reflective and serve as thereflector in the resulting interference structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a simplified cross section diagram of a multi-layerinterference structure according to an embodiment of the presentinvention.

FIG. 1B shows the calculated performance of two different structures asa function of absorber thickness.

FIG. 1C shows the calculated performance of two different structures asa function of absorber thickness.

FIG. 2A is a simplified cross section diagram of a portion of a pigmentflake according to an embodiment of the present invention.

FIG. 2B is a simplified cross section diagram of a pigment flakeaccording to an embodiment of the present invention.

FIG. 2C is a simplified cross section of a pigment flake according toanother embodiment of the present invention.

FIG. 2D is a simplified cross section of a pigment flake according toanother embodiment of the present invention.

FIG. 2E is a simplified cross section of a pigment flake according toanother embodiment of the present invention.

FIG. 3A is a simplified cross section diagram of a multi-layerinterference structure with a center thickening layer according to anembodiment of the present invention.

FIG. 3B is a simplified cross section of a pigment particle according toanother embodiment of the present invention.

FIG. 3C is a simplified cross section of a portion of a pigment particleaccording to another embodiment of the present invention.

FIGS. 4A and 4B are graphs of the predicted reflectance versuswavelength for an embodiment of the present invention.

FIGS. 5A and 5B are graphs of the measured reflectance versus wavelengthfor an embodiment of the present invention.

FIGS. 6A and 6B are graphs of the predicted reflectance versuswavelength for another embodiment of the present invention.

FIGS. 7A and 7B are graphs of the predicted reflectance versuswavelength for another embodiment of the present invention.

FIGS. 8A-8C are simplified cross sections of films and foils accordingto embodiments of the present invention.

FIGS. 9A and 9B are simplified cross sections of polymeric sheetsaccording to embodiments of the present invention.

FIG. 10 is a simplified cross section of a paint layer applied to asurface according to an embodiment of the present invention.

FIG. 11 is a simplified plan view of an image according to an embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to optical thin film structures thatprovide low perceived color and relatively high IR reflectivity.Embodiments of the invention may be embodied as pigment flakes for usein paints, inks, plastic sheets (films), plastic objects, and powdercoatings, as foils that are attached to objects, or coatings that areformed directly on objects. In some embodiments, the optical thin filmstructures appear very dark or even black. Objects coated with pigments,foils, or coatings according to these embodiments of the invention havelow perceived color because of the low lightness, even though themeasured chroma may be fairly high. In other embodiments, a pigment,foil, or coating appears gray, which may have high lightness incombination with relatively low chroma. In yet other embodiments, theoptical thin film structures can be selected to provide a low level ofcolor. Typically, such optical thin film structures exhibit littlegoniometric color shifting, unlike what are commonly known as opticallyvariable pigments.

Table 1 briefly outlines some of the various embodiments of the presentinvention. The average reflectance from 400-700 nm is basically ameasure of the lightness (L*) in the visible region. The averagereflectance between 4-40 micrometers is basically an indication of theIR reflectance, and the chroma (C*_(ab) according to CIE™ colormetricconventions) is basically an indication of the visible colorcharacteristic of the surface or object. A high IR reflectance isdesirable to reduce IR emissivity, such as for solar energy collectorsor IR camouflage. TABLE 1 Comparison of Optical Attributes Between SomeEmbodiments of the Present Invention Ave. Reflectance Ave. ReflectanceAppearance 400-700 nm 4-40 microns Chroma (C*_(ab)) Black <20% >50% upto 50 Gray <50% >50% <5 Low Color <50% >50% <20

Other types of pigments are designed to provide high chroma and a highdegree of optical variance, i.e. color change, with viewing angle.However, in certain embodiments of the invention a relatively neutralcolor of the pigment flakes arises from a substantially constantreflectivity as a function of wavelength within the visible spectralregion. A change in viewing angle does not produce a significant colorchange because the spectral response remains relatively flat at shorterwavelengths.

Black, gray, and low-color optical thin film structures according tosome embodiments of the present invention are relatively thin, comparedto multi-layer high-chroma optical thin-film stacks, for example. Whendepositing thin designs (e.g. optical thin film stacks having a totalthickness less than 250 nm) onto flakes, it may be desirable to providea stiffening layer(s) or to form the optical design on stiff flakes,such as glass flakes, which are discussed in further detail below.

I. General Description of Optical Thin Film Structures According toEmbodiments of the Present Invention

FIG. 1A is a simplified cross section of a multi-layer interferencestructure 10 according to an embodiment of the present invention. Thestructure includes an optical thin film stack 15 deposited on asubstrate 11. The substrate could be a flake, such as a glass or metalflake, a foil or film, or an object, such as a structural panel. Onlyone side of the substrate is shown with an optical thin film stack, butthe opposing side of the substrate could also have an optical stack,being the same as or different from the one illustrated. Flakes areoften coated symmetrically on both sides, as further discussed inrelation to FIGS. 2A and 2B, below.

The first layer 12 of optical stack 15 is reflective in at least theinfrared wavelength range of interest. A spacer layer 13 overlies theIR-reflective layer 12. The spacer layer can be either a dielectric orsemiconductor material, but in this embodiment is a dielectric material.A layer of absorber material 14 is coated onto the spacer layer 13. Thespacer and absorber layers are in optical communication with each otherand the underlying reflective layer in the sense that they form anoptical interference coating in which the incident light is selectivelyattenuated by destructive interference and/or induced absorption. Itwill be recognized by those skilled in the art of optical coating designthat a single layer can be replaced by a plurality of multiple layerswithout substantially affecting the function, that is the opticalcommunication of the aforementioned layers as set forth above.Similarly, the substrate itself may be reflecting and act as areflector. In that case the separate reflective layer might then beomitted. Thus, the recitation of a single layer in optical communicationwith adjacent layers is intended to encompass such replacements andsubstitutions.

The optical stack 15 may comprise additional layers ofdielectric-absorber (D-A) layer pairs. The D-A layer pairs may comprisethe same sets of materials or different materials; for example layersequence R/D₁/A₁/D₂/A₂, where R is the reflector layer and thesubscripts 1 and 2 denote different materials in the classes of spacersand absorbers. The stack can have other structures, for example a layersequence R/D₁/A₁/D₂ in which the absorber layer is overcoated or inoptical communication with a second dielectric layer D₂. When thethickness of D₂ is less than about 8 times that of layer D₁, layer D₂affects and can enhance the performance of the optical stack 15, and actas an anti-reflective coating in some instances. At thicknesses aboveabout 8 times layer D₁, layer D₂ is used primarily as a protectiveovercoat.

The thickness of the overcoat may be selected according to the intendedapplication. For example, if the optical thin film structure is formedon a pigment flake, the overcoat might be relatively thick for use in apaint formulation, and relatively thin for use in an extruded plasticsheet. The thickness of the overcoat may be selected according to theimproved rigidity it provides so that the pigment flakes can withstandhigh-stress processes, and/or according to the desired protection fromthe environment. A flake intended for use in a carrier that provides ahigh degree of protection from the environment, or for use in arelatively benign environment, might have a thinner overcoat or noovercoat, while a flake intended for use in a harsh environment mighthave a more substantial overcoat.

The design of the optical stack, which is primarily the thicknesses andcompositions of the reflector, dielectric and absorber layers, can bevaried according to the teachings of the invention to selectivelycontrol the visible color and the infrared properties. The thickness ofthese layers may be characterized by a physical thickness, t, in forexample micrometers, or by the quarterwave optical thickness (“QWOT”)with respect to light of a particular wavelength, λ, wherein n is therefractive index of the layer at designated wavelength:QWOT=t/4×n(λ)  (Eq. 1)

While the optimized thickness of these layers will depend on theiroptical properties, such as refractive index and extinction coefficientat visible and infrared wavelengths, the thickness of the absorbermaterial is generally selected to control the visible reflectivity ofthe stack based on the desired visible and infrared performance. As thethickness (t_(a)) of the absorber material increases from zerothickness, the optical stack reflectivity decreases from a valuecharacteristic of the reflective layer, R_(r) (λ_(vis)) to a minimumvalue. This reflectance drop is due to increasing levels of destructiveinterference between light beams leaving the reflector layer and thoseleaving the partially-reflecting absorber layer. Further increases inabsorber thickness cause the device's visible reflectance to increasefrom the minimum value to a higher value consistent with the inherentreflectivity of the absorber material. In some embodiments, thesubstrate can be reflective, and the separate reflective layer can beomitted. Similarly, in some embodiments the reflective minimum liesoutside the visible range, such as at 850 nm in the near IR or at 300 nmin the near ultraviolet (“UV”). These wavelengths are merely exemplary.

FIGS. 1B and 1C illustrate the calculated performance of differentstructures as a function of absorber thickness for an optical stackusing an aluminum reflector, which is relatively bright, and an opticalstack using a chromium reflector, which is not as reflective asaluminum. The curve 26 shown as a dotted line represents the predictedreflectance at 510 nm of an aluminum reflector with one quarter-wave (at470 μm) thickness of magnesium fluoride, and a chromium absorber. Thecurve 28 shown as a solid line represents the predicted reflectance at510 nm of a chromium reflector with one quarter-wave (at 460 nm)thickness of magnesium fluoride, and a chromium absorber. The spacerthickness may be greater or less than the examples, and a change in thespacer thickness sometimes results in an adjustment to the thickness ofthe absorber layer to achieve minimum reflectance in the visiblespectrum.

FIG. 1C illustrates that as the absorber layer gets thicker, thereflectivity of the stacks rises from the minimum until it reaches thereflectivity of the absorber material itself. Stated differently,devices with infinitely thick absorbers no longer act as interferencedevices but instead have the appearance and performance of the absorbermaterial across both the visible and infrared portions of the spectrum.Although a reflectivity minimum of about zero is attained with eachstack, the optimum value of the absorber layer thickness for an opticalstack using a relatively dark reflector is less than the optimumabsorber thickness for an optical stack with a brighter reflector.Similarly, the optimum thickness of the spacer layer used with thedarker reflector also decreased slightly.

The absorber thickness depends upon the desired levels of visiblereflectance and infrared reflectance. For the device to have a darkappearance and a high far-infrared reflectance, the thickness of theabsorber should provide the minimum or near-minimum reflectance in thevisible region. This value will be called tabs_(min) (see FIG. 1B, ref.num. 29) for the optical stack with the aluminum reflector. When aninfrared reflectance more like the opaque absorber material (generallylower) is desired, the thickness of the absorber should be greater thantabs_(min). Conversely, when the visible reflectance of the device isdesired to be closer to that of the bare reflector, the thickness of theabsorber should be less than tabs_(min). In the range of about0-3×tabs_(min) the visible performance of the device can be selectedwithout significantly degrading the far-infrared thermal performance ofthe device. The optimal absorber thickness for other optical stacks maybe different depending on the type of reflector and spacer thickness.

The materials for the reflector layers are selected to have thereflective characteristics suitable for the intended use of the foil orpigment. A preferred reflector material is aluminum, which has goodreflectance characteristics, inexpensive, and is easy to form into athin layer. It will be appreciated in view of the teachings herein,however, that other reflective materials may be used in place ofaluminum, such as silver, iron, tantalum, iridium, rhenium, copper,silver, gold, platinum, palladium, nickel, cobalt, niobium, chromium,tin, and combinations or alloys of these or other metals or othermaterials that reflect in the infrared spectrum of interest. Otheruseful reflective materials include, but are not limited to, thetransition and lanthanide metals and combinations thereof; as well asmetal carbides, metal oxides, metal nitrides, metal sulfides,combinations thereof, or mixtures of metals and one or more of thesematerials. Accordingly, specific examples of suitable IR reflectingmaterials include indium oxide, indium tin oxide (ITO), europium oxide(Eu₂O₃), vanadium pentoxide (V₂O₅), rhenium oxide (ReO₃), lanthanumboride (LaB₆), combinations thereof, and the like. The thickness of thereflective layer is selected so that it is at least semi-reflective, butpreferably opaque at infrared wavelengths. The reflectivity of thereflective layer is preferably greater than 50% over the wavelengthrange of 4 to 40 micrometers, and the thickness of the reflective layercan be increased to improve the stiffness of pigment particles orimprove handling.

The spacer layers 23 a and 23 b are typically made of materials havingindices of refraction in the range from about 1.2-4.5. The spacer layerscan be composed of various materials such as those having a “high”refractive index, i.e. greater than about 1.65. Nonlimiting examples ofsuitable high index materials include zinc sulfide (ZnS), zinc oxide(ZnO), zirconium oxide (ZrO₂), titanium dioxide (TiO₂), diamond-likecarbon, indium oxide (In₂O₃), indium-tin-oxide (“ITO”), tantalumpentoxide (Ta₂O₅), ceric oxide (CeO₂), yttrium oxide (Y₂O₃), europiumoxide (Eu₂O₃), iron oxides such as (II)diiron(III) oxide (Fe₃O₄) andferric oxide (Fe₂O₃), hafnium nitride (HfN), hafnium carbide (HfC),hafnium oxide (HfO₂), lanthanum oxide (La₂O₃), magnesium oxide (MgO),neodymium oxide (Nd₂O₃), praseodymium oxide (Pr₆O₁₁), samarium oxide(Sm₂O₃), antimony trioxide (Sb₂O₃), silicon (Si), silicon monoxide(SiO), germanium (Ge), selenium trioxide (Se₂O₃), tin oxide (SnO₂),tungsten trioxide (WO₃), combinations thereof, and the like. Othersuitable high index materials include mixed oxides. When used as spacerlayers, materials are most commonly oxidized to their stoichiometricstate, such as ZrTiO₄, but may be sub- or super-oxidized. Non-limitingexamples of such mixed oxides include zirconium titanium oxide andniobium titanium oxide.

Some spacer materials exhibit absorption in the far infrared. Becausethe spacer layers are relatively thin, generally only one or twoquarter-waves in optical thickness in the visible spectrum, inclusion ofspacer layers with IR absorbing material does not unduly affect the IRemittance of devices.

Spacer layers can be either selective or non-selective in the visibleregion. When selectively absorbing in the visible, the spacer workstogether with the absorber layer via optical interference to modify thereflected color, generally attenuating the reflection over thewavelengths where the spacer absorbs. This combinatory effect providescolor shades that are not otherwise available with optical interferencestructures having non-selective spacer layers. At the same time, thestructures can provide high infrared reflectance. Examples ofselectively absorbing spacer materials include iron oxide, tungstenoxide, copper oxide, and cobalt oxide.

The spacer layers can each be composed of the same material or differentmaterials, and can have the same or different optical or physicalthickness for each layer. It will be appreciated that when the spacerlayers are composed of different materials or have different thicknessesthe pigment flakes exhibit different colors on each side, and theresulting mix of flakes in a pigment or paint mixture would show a newcolor that is the combination of the two colors. The resulting colorwould be based on additive color theory of the two colors coming fromthe two sides of the flakes. In a multiplicity of flakes, the resultingcolor would be the additive sum of the two colors resulting from therandom distribution of flakes having different sides oriented toward theobserver.

When a high level of visible absorption (dark appearance) is desired,the spacer layer preferably has an optical thickness of about 1.0 QWOTat 200 nm to about 2.0 QWOT at 500 nm, and most preferably about 1.0QWOT at 300 nm to about 1.0 QWOT at 700 nm. For maximum solarabsorbance, the refractive index of the spacer layer is more preferablyless than 2.0, most preferably less than about 1.65. Use of a low indexdielectric material broadens the wavelength region of low reflectance,thus increasing the level of solar absorption. Examples of low-indexmaterials include silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), metalfluorides such as magnesium fluoride (MgF₂), aluminum fluoride (AlF₃),cerium fluoride (CeF₃), lanthanum fluoride (LaF₃), sodium aluminumfluorides (e.g., Na₃AlF₆ or Na₅Al₃F₁₄), neodymium fluoride (NdF₃),samarium fluoride (SmF₃), barium fluoride (BaF₂), calcium fluoride(CaF₂), lithium fluoride (LiF), combinations thereof, or any other lowindex material, i.e. a material having an index of refraction of about1.65 or less. For example, organic monomers and polymers can be utilizedas low index materials, including dienes or alkenes such as acrylates(e.g., methacrylate), perfluoroalkenes, polytetrafluoroethylene (e.g.TEFLON®), fluorinated ethylene propylene (“FEP”), combinations thereof,and the like.

It should be appreciated that several of the above-listed dielectricmaterials are typically present in non-stoichiometric forms, oftendepending upon the specific method used to deposit the dielectricmaterial as a coating layer, and that the above-listed compound namesindicate the approximate stoichiometry. For example, silicon monoxideand silicon dioxide have nominal 1:1 and 1:2 silicon:oxygen ratios,respectively, but the actual silicon:oxygen ratio of a particulardielectric coating layer varies somewhat from these nominal values. Suchnon-stoichiometric dielectric materials are also within the scope of thepresent invention.

Suitable materials for the absorber layer include metals, alloys, andcompounds that can be reliably deposited at a thickness at which theyare semi-transparent at visible wavelengths. The semi-transparent layermay have either uniform absorption, i.e. neutral density, or selectiveabsorption across the wavelengths of the visible spectrum, or selectiveabsorption between the visible and IR regions, depending on the desiredcoloration and the color of the native reflector layer. It should beunderstood that the absorber coating at this thickness does not need tobe continuous to still work as an optical absorber. For example, aplurality of islands or dots of absorber material can suffice as anabsorber. Examples of suitable metals include chromium, nickel, iron,titanium, aluminum, tungsten, molybdenum, niobium, combinations,compounds or alloys thereof, such as INCONEL™ (Ni—Cr—Fe), metals mixedin a dielectric matrix, or other substances that are capable of actingas a uniform or selective absorber in the visible spectrum.Alternatively, the absorber can also be a dielectric material such as aniron oxide (e.g., Fe₂O₃), silicon monoxide (SiO), chromium oxide(Cr₂O₃), carbon, titanium nitride (TiN), titanium sub-oxide (TiO_(x)where x is less than 2.0), combinations thereof, and the like.

FIG. 2A is a simplified cross section of a pigment particle 20 with asymmetrical multi-layer interference structure according to anotherembodiment of the present invention. The spacer layers 23 a, 23 b, andabsorber layers 24 a, 24 b, are symmetrical about the reflector 22. Thediameter of the pigment particle (parallel to the plane of the layers)is typically about 5 microns to about 100 microns, but preferably fromabout 10 microns to about 40 microns, and may be any of a variety ofshapes, including irregular shapes. Although the particle is shown as anessentially planar stack, flat layers are not required. In someinstances, the particle can take on an irregular shape, either as seenin a plan view, or cross section. In a particular embodiment, theparticle is shaped similarly to a lima bean. The deposited layers aregenerally locally parallel to the surface on which they are deposited.Some techniques of layer formation on flakes, such as sol-geltechniques, tend to create rounded particles.

The pigment particle 20 has a central reflective layer 22. Thereflective layer has dielectric or other spacer layers 23 a and 23 b oneach major surface, with absorber layers 24 a and 24 b deposited on thespacer layers. Pigment particles according to embodiments of the presentinvention may include additional layers of dielectric-absorber pairs.For example, layer sequence A₂/D₂/A₁/D₁/R/D₁/A₁/D₂/A₂ uses twodielectric-absorber pairs D₁-A₁ and D₂-A₂. The D-A layer pairs may havethe same materials or different materials on each side of the reflectivelayer. One side of the pigment particle might have a different number oflayers than the other, for example A₁/D₁/R/D₁/A₁/D₂/A₂. Similarly, thelayers may be of different materials, such as A₁/D₁/R/D₂/A₂ orA/D₁/R/D₁/A₂, etc., or corresponding layers on opposite sides of thereflective layer might be of different thicknesses. The pigment particle20 may have other structures, for example layer sequenceD₂/A₁/D₁/R/D₁/A₁/D₂ in which the absorber layers 24 a and 24 b would beovercoated with additional dielectric layers (not shown). In someinstances, it may be desirable to overcoat an optical stack with adielectric layer that does not significantly alter the opticalproperties of the stack, but provides environmental protection to thestack.

When the thickness of such an overcoat dielectric layer is less thanabout 8 times that of layer of the first dielectric layer 23 a, theovercoat dielectric layer affects and can enhance the performance of thepigment particle, in some cases acting as anti-reflective layer. Atthicknesses above about 8 times the thickness of the first dielectriclayer, the overcoat dielectric layer is used primarily as a protectiveovercoat.

FIG. 2B is a simplified cross section of a pigment flake 21 according toan embodiment of the present invention showing the central reflectivelayer 22 surrounded by a spacer layer 33 and an absorber layer 34. Thecentral reflective layer could be a rigid flake with a reflective thinfilm layer deposited on both sides, or a metal flake, for example.

FIG. 2C is a simplified cross section of a pigment particle 110according to another embodiment of the present invention. The pigmentparticle has improved rigidity and handling characteristics achieved bythe addition of one or more central stiffening or thickening layers 112.The particle also includes reflector layers 114 a and 114 b that formpart of the optical stacks 116 a and 116 b, which include spacer andabsorber layers (not individually shown). The stiffening layer can bemade from a wide variety of different materials including metals,alloys, dielectrics, and semiconductors.

Stiffer particles are desirable because they better survive higher-shearprocesses used in spray painting and because their stiffness allows foreasier size-reduction processes (i.e. milling or grinding), whilemaintaining the flatness of the particle. Maintaining flatness isdesirable so that the aggregate effect of the particles applied to asurface is similar to the optical characteristics of a foil with asimilar optical thin film stack applied to the surface. The stiffeninglayer typically has a thickness in the range of 10 nm to 101 m,preferably between 200 nm and 2 μm, which can improve handling, such assizing and dispersing, in addition to stiffening the particles. Thereflector layers 114 a and 114 b can be of the same types as reflectorlayers 12 and 22 described above. Similarly, it should be appreciatedthat optical stacks 116 a and 116 b can be of the same types as opticalstacks described above in relation to FIGS. 1A and 2A, and can besymmetrical or non-symmetrical.

FIG. 2D is a simplified cross section of a pigment flake 60 according toanother embodiment of the present invention. The pigment flake 60 has acentral pre-flake 61. The pre-flake 61 has a central layer or core 61 aformed of a reflective material onto which are disposed layers oftransparent material 61 b, 61 b′ having a sufficiently high stiffness toreinforce the reflecting layer and make the pre-flake 61 substantiallyrigid. The core 61 is aluminum about 30 nm thick, which is generallyopaque, but could be other materials or thicknesses. The transparentlayers 61 b, 61 b′ are SiO_(x), (where x is less than 2.0) about 10-50nm thick, but could be other materials. These silicon oxide layers aretransparent and provide stiffness in conjunction with the aluminumreflective layer.

A dielectric layer 62 is then coated on the preflake 61, followed bydeposition of an absorber layer 63. The dielectric and absorber layersare coated on all sides of the preflake, including its ends. Thethickness and material of the dielectric layer 62 is selected takinginto account the optical thickness of transparent layers 61 b and 61 b′to provide low chroma.

FIG. 2E is a simplified cross section of a pigment flake 70 according toanother embodiment of the present invention. The dielectric layer 72 isless than about 250 nm in thickness. A reflective layer 71 a isdeposited on a stiff flake 71, which may be desirable in combinationwith the relatively thin dielectric layer to provide a stiff pigmentparticle. Rather than incorporating a central vacuum-depositedstiffening layer or forming a multi-layer pre-flake by a vacuumdeposition method (and subsequent stripping and grinding to produceappropriately sized flakes) the thin film layers are deposited insequence on a relatively flat and substantially rigid particle, such asglass, mica, alumina, iron oxide, graphite, bismuth oxychloride, boronnitride, polymer or metal or similar particle. By coating a relativelyflat flake 71 with a reflective layer 71 a, one creates a pre-formedplatelet-like flake suitable for further coating, similar to thepreflake 61 discussed in accordance with FIG. 6. In this case, thereflective layer surrounds the stiff flake. Examples of methods forcoating such relatively flat and substantially rigid particles arechemical vapor deposition (CVD), physical vapor deposition (PVD),including sputtering, and electroplating. A spacer layer 72 is depositedto surround the reflective layer 71 a, and an absorber layer 73 isdeposited to surround the spacer layer.

FIG. 3A is a simplified cross section of a pigment flake 80 according toanother embodiment of the present invention. The relative thicknesses ofthe layers are not shown to scale. A reflective preflake 81 includesreflective layers 81 b, 81 b′, separated by a rigid layer 81 a. Thereflector layers 81 b, 81 b′ are typically opaque or nearly opaque;therefore, the optical properties of the rigid layer are not important.The material and thickness of the rigid layer are selected according tothe overall thickness and mechanical properties of the other thin filmmaterials in the multi-layer pigment, which include a spacer layer 82and an absorber layer 83. The rigid layer could be made of metal,dielectric material, semiconductor material, or organic material.

The thickness of the rigid layer is typically between about 10 nm to 10μm, more preferably from about 100 nm to about 2.0 μm and mostpreferably from about 200 nm to about 500 nm, depending on the materialof the rigid layer and the intended deposited layers.

In a particular embodiment, the rigid layer 81 a is a layer of siliconmonoxide (SiO or SiO_(x) where x<2) about 250 nm thick covered on bothsides with reflector layers 81 b, 81 b′ of opaque iridium. Increasingthe total thickness of the thin film structure in this manner aids instripping the materials off a plastic web substrate, such as is used inroll-coating techniques, to form preflakes or platelets for pigmentsaccording to embodiments of the invention. Additionally, thicker pigmentparticles were found to be easier to sort (size) and handle compared tooptically equivalent, yet physically thinner designs.

When such a multilayer thin film interference coating is stripped from asupporting substrate or web, the flakes or platelets typically range insize (distance across the face) from 2-200 μm. The platelets can befurther reduced in size as desired. For example, the flakes can besubjected to an air grind to reduce their size to a size rangingtypically from about 2-50 micrometers without adversely affecting theiroptical characteristics. The flakes or platelets are produced to have anaspect ratio of at least 2:1 and preferably about 5:1 to about 15:1,with a narrow particle size distribution. The aspect ratio isascertained by taking the ratio of the largest dimension of a surface ofthe flake parallel to the planes of the layers forming the thin film tothe thickness dimension of the platelet.

FIG. 3B is a simplified cross section of a pigment particle 84 accordingto another embodiment of the present invention. In this case thepreflake includes a rigid layer 81 a, reflector layers 81 b, 81 b′, andspacer layers 85, 85′. The thin film layers can be roll-coated onto afilm and then removed and processed into platelets of desired size, asdiscussed in conjunction with FIG. 3A, above. The absorber layer 86surrounds the preflake.

FIG. 3C is a simplified cross section of a pigment particle 88 accordingto another embodiment of the present invention. A rigid layer 81 a,reflective layers 81 b, 81 b′, spacer layers 85, 85′ and absorber layers87, 87′ have all been formed on film and then removed and processed intoplatelets of desired size. The rigid layer is sufficiently thick toprovide the desired handling characteristics.

In order to impart additional durability to interference pigment flakesaccording to embodiments of the present invention, it is sometimesdesirable to anneal or heat treat the platelets at a temperature rangingfrom about 200° C. to about 300° C., preferably from about 250° C. toabout 275° C., for a period of time ranging from 10 minutes to 24 hours,preferably about 15-30 minutes in air or an inert atmosphere, such as N₂or Ar.

It should be appreciated that although FIGS. 2A, 2B, and 2E showstructures in which each layer completely surrounds the layerunderneath, devices with essentially equivalent performance can beachieved with layers that do not surround those layers underneath, suchas shown in FIGS. 2C, 2D, and 3A-3C. Similarly, while optical stacksaccording to the invention have been illustrated with singlespacer-absorber layers, multiple spacer-absorber layer pairs could beused. In some embodiments, a functional layer may be made up of morethan one material. For example, a thin, non-opaque highly reflectivelayer might be formed on a less-reflective particle, or the spacer layermight be made up of multiple layers of dielectric material, as discussedabove in conjunction with FIG. 6A.

The pigment flakes described in accordance with FIGS. 1A, 2A-2E, and3A-3C can also be manufactured using a PVD process to deposit thelayers. A collection of flakes can be produced by forming a symmetricalmulti-layer thin film structure on a flexible web of material andseparating the thin film structure from the web to provide a collectionof platelets. While some pigment flakes made according to embodiments ofthe present invention using roll coating techniques, particularlyrelatively thin flakes, perform well optically, they can be difficult tostrip from the plastic web and to process down to size desirable for usein paints and inks without a stiffening layer. Other embodiments areeasily stripped from the plastic web. In some applications, the desiredparticle size is about 20 μm. In other applications, it may be desirableto use particles in the range of about 5 microns to about 100 micronsacross.

Although highly reflective particles according to some embodiments ofthe present invention can have low chroma, it is generally desirablethat optical stacks according to the present invention have an averagereflectance less than about 50% in the visible spectrum. Chroma(C*_(ab)) is generally the degree of perceived color of an objectcompared to a gray object having the same lightness (L*) and is furtherdefined according to the CIE 1976 L*a*b* color system by the formulaC*_(ab)=(a*²+b*²)^(0.5). Even objects with low average reflectivity canat the same time have a non-uniform reflectivity imparting color, i.e.chroma, to the object. Thus it is further desirable that the chroma ofan optical stack, or pigment particles or foils incorporating opticalstacks according to embodiments of the present invention have a chromaless than about 20. Other embodiments, such as foils, surfaces, orpigment particles that appear black, may have higher chroma because thelightness is so low, typically less than about 5. In these embodimentschroma may be as high as 50 without imparting significant perceivedcolor.

II. An Optical Thin Film Structure with a Black Appearance

In a multilayer interference structure of an embodiment, performancefactors such as infrared emittance, solar absorbance, and visible colorvalues are determined by the choice of number of layers, the materialsand their thicknesses. When a black visible appearance is desired, theabsorber thickness is chosen in combination with the reflector accordingto the absorber material to reduce the reflectance of the optical stackin the visible spectrum. For example, if the reflector layer isaluminum, a preferred absorber has a visible transmittance of about 50%,which is considered moderate transmittance, to reduce the averageresulting visible reflectance of the optical stack to less than about20%. In contrast, if the reflector is chromium, tantalum, or anothermaterial having a lower reflectivity than aluminum, the absorber layertransmittance may be increased in correspondence to the lower visiblereflectivity of the reflector layer.

FIG. 2B represents one type of pigment flake configuration that could beused for embodiments of the present invention. This type of flake issymmetrical, in other words, the coating layers are essentially the sameon all sides. Such symmetry is not required. For example, flakes mighthave symmetry only with regards to the layers on the major surfaces, ormight have different thin film layers on different surfaces.

In one embodiment, a pigment flake in accordance with FIG. 2B has acentral reflector layer of aluminum about 100 nm thick, a dielectriclayer of MgF₂ having a thickness of about 1.0 QWOT @ 480 nm to provideminimum reflectance in the visible region, and an absorber layer ofchromium with a thickness of about 6 nm to provide about 50%transmission through the as-deposited chromium layer.

The absorber thickness is chosen to give the minimum reflectance overthe visible spectrum, with the minimum reflectance centered around 510nm. The design appears black to the human eye, with an anticipatedlightness (L*) of about 6 and a chroma (C_(ab)*) of about 50.

A flake-like pigment according to such a design is preferably mixed intoa low emittance polymer binder and applied like paint, turning surfacesinto a high efficiency solar absorber. FIG. 4A illustrates the predictedvisible performance of the device described above, while FIG. 4B showsthe predicted performance of the same design over the visible andinfrared wavelengths.

When used as a pigment, this design has several possible advantages overtraditional absorptive black pigments. For example, decorative blackpaints according to embodiments of the present invention can achieve ablack metallic (“sparkly”) effect. It is believed that the sides of theflakes provide relatively strong localized reflection in the visibleregion, thus providing the sparkle. Alternatively the black pigmentflakes could be mixed with bright (reflective) flakes in a clear carrier(paint base) to provide a gray sparkle finish. In conventional paintsystems, a black color is typically obtained by mixing fine carbon blackor similar pigment into the paint, which dulls reflectivity occurringfrom reflective flakes.

One can achieve dark colors by changing the dielectric thickness of theinterference design. Increasing the dielectric thickness gives colors ofindigo and dark blue; decreasing the thickness gives dark brown. Thewavelength of minimum reflectance decreases (toward blue/UV to values inthe range of 200-510 μm) with decreasing dielectric thickness andincreases (toward red/IR to values in the range of 510-1500 nm) withincreasing dielectric thickness.

The design's reflectance is higher in the infrared than in the visibleportion of the spectrum, thus able to provide a covert feature in asecurity device. When placed in outer space or in outdoor locations, thedesign provides simultaneous high absorption of solar wavelengths(0.2-2.5 μm) and low emittance of thermal wavelengths (4-40 μm) makingit an excellent solar absorber.

An optical design as described above for the pigment flake discussed inconjunction with FIGS. 4A and 4B can be adapted for deposition onto afoil. The foil can be laminated, glued, or otherwise applied to anotherobject, such as a solar absorber module, to provide that object withenhanced optical absorption and thermal retention.

III. An Optical Thin Film Structure with a Gray Appearance

When a gray appearance is to be coupled with a moderate infraredemittance (moderate infrared reflectance), the transmittance of theabsorber is chosen to be lower than for the black case. For example,when the reflector is aluminum, a preferred absorber has a visibletransmittance of about 15% to about 50% depending on the desiredlightness, while the visible transmittance can be as low as 5% and stillfunction in this manner. So now we have a desired absorber thicknessproviding between 15-85% transmittance for a reflector with not lessthan 80% reflectance, depending upon the degree of infrared emittancethat is desired. This selection allows one to create optical stacks withsimilar visual characteristics, yet different IR characteristics.

In another embodiment of the present invention, a pigment flake inaccordance with FIG. 2B has a central reflector layer of aluminum about80 nm thick, a dielectric layer of MgF₂ with an optical thickness ofabout 1.0 QWOT @ 500 nm, and an absorber layer of chromium with aphysical thickness of about 20 nm. In this design, the absorber anddielectric thicknesses are chosen to give a medium level of lightness(L*), with nominally uniform reflectance over a range of wavelengths, orin other words, a gray visible appearance. This embodiment provides ahigh level of reflectance in the infrared. Devices of this type areuseful in several applications. For example, if some but not all of thesun's energy is to be absorbed and retained. Another example is whenboth visible and IR camouflage is desired for objects in the sky or onthe sea. A gray color is useful for the visible camouflage, while thehigh IR reflectance prevents high emission of thermal energy that couldbe observed by IR detectors.

This embodiment can provide advantages for military applications, wherenight vision type thermal imaging scanners are used to detect articlesor objects warmer than their surroundings. An article coated with eithera foil or pigment-containing coating according to embodiments of thepresent invention will emit less thermal infrared energy than itsuncoated counterparts, rendering it less distinguishable fromsurrounding objects at night, as well as providing visible colorationcompatible with daytime camouflage. Articles painted with traditionalgray paints pigmented with mixtures of carbon black and white TiO₂,generally do not have significantly suppressed IR emissions.

In general, simultaneous visual and IR camouflage of objects from humanand artificial surveillance and tracking devices can be obtained with anopaque exterior coating having a low luminous reflectance, low solarabsorption, and low infrared emittance. In other embodiments, a dark ormoderately dark exterior coating having a low luminous reflectance andhigh infrared reflectance is desired. The exterior may serve as visualcamouflage, either as a solid color or as patterned colors.

FIGS. 5A and 5B are graphs illustrating the measured reflectivity versuswavelength for an optical stack in accordance with the example of thegray optical structure discussed above. The reflectance of the devicereaches a minimum value around 1.3 μm in the near infrared and thenincreases in the mid and far-infrared regions. The dielectric thicknesscan vary from a low value such as 1.0 QWOT @ 200 nm to a higher valuesuch as 1.0 QWOT @ 800 nm while still providing a nominally grayappearance.

Varying the thickness of the absorber layer can modify the reflectanceprofile. When the absorber thickness is decreased from the thickness ofthe absorber layer that provides the minimum reflectance (tabs_(min),which is about 50% transmittance for an aluminum reflector), the pigmentappears gray, rather than black. For example, with an optical stackhaving a minimum reflectance at 510 nm, decreasing the absorberthickness increases the transmittance through the absorber layer,resulting in a progressive change in appearance from black to dark grayto medium gray to light gray and finally to silver, at about 85%transmittance through the absorber layer. Thus, the lightness value, L*,is selectable based on the absorber thickness going from a value nearzero when ta=tabs_(min), increasing steadily to the value of the barereflector as ta approaches zero. All this while the mid- and far-IRreflectance remains high. The choice of the reflector can determine thefinal color or shade of gray. Foils or pigments with highly reflecting(>80%) metal as the reflector layer, such as aluminum or silver,approach their native appearance, i.e. a silver color, with very thinabsorber layers. The use of metals or other materials with lowerreflectivity, such as iron, silicon or tantalum, as the reflector layerlimits the color to grays, even with very thin absorber layers.

When the absorber thickness is increased from the thickness of theabsorber layer that provides the minimum reflectance, the pigment alsoappears grayish. However, as the absorber transmittance is decreasedfrom about 50% to 0% (increasing absorber thickness) the optical stackprogressively ceases to act as an interference structure. The appearanceof the optical stack approaches that of the absorber layer, changingfrom black through dark gray to medium gray, as the absorber layerbecomes substantially opaque. As the transmittance of the absorber layerdecreases from about 50% to 0%, the infrared emittance is also affectedand progressively approaches that of the opaque absorber material.Devices in this range are very useful when it is desirable to have botha moderate level of solar absorbance along with a moderate level ofemittance.

IV. Low-Color Optical Thin-Film Structures

When a neutral appearance is desired such as in the gray and blackexamples above, the optical design of layer construction, materials, andthicknesses is chosen to minimize the absolute chroma. There are otheroptical designs that provide low color purity and chroma but have someperceived color. For example, the optical design that gives the maximumsolar absorbance is not always the design that provides a blackappearance. In other cases, a pigment or foil with a slightly, typicallydull, colored appearance might be desired for aesthetic reasons. Some ofthese dull colors are variants of gray such as gray-green, gray-blue,and steel gray. The low-chroma blue design, for example, is suitable forapplications where it is desirable to avoid visible detection. Othercolors can be considered variants of black such as dark brown, darkblue, deep burgundy, dark green, and indigo. These colors can beobtained using variations of the asymmetrical and symmetricalembodiments above and at the same time provide the desired levels ofsolar absorbance and infrared emittance.

Typically, to obtain thermal control structures with a dull but coloredappearance, the dielectric (spacer) thickness is increased or decreasedfrom that which gives neutrality (i.e. no perceptible color). Startingfrom a neutral black, as the dielectric thickness increases, the colortypically changes from black to indigo to dull blue to dull green. Asthe dielectric thickness decreases, the color usually moves from blackto dark brown.

The dielectric or spacer layer for colored low-chroma optical stackspreferably comprises materials having indices of refraction in the rangefrom about 1.2 up to 4.5 and preferably has an optical thickness ofbetween about 1 QWOT at 200 nm and 2 QWOT at 700 nm, and more preferablyabout 1 QWOT at 300 nm to about 2 QWOT at 500 nm. The refractive indexof the dielectric layers is more preferably greater than 1.65, mostpreferably greater than about 2.0. Selection of a high refractivedielectric material is one way to minimize the variation in interferencecolor with angle incidence, that is, a shift in observed color as theobserver changes his position with respect to the coated object, orviews a plane or portion of the objected disposed at a non-parallelorientation with respect to other portions of the object or article(assuming a fixed light source). In comparison, optical stacks used inapplications where color shifting is desirable typically use spacerlayers made of materials with a refractive index less than 1.65, andmore preferably less than about 1.5.

The color shifting tendencies of optical stacks according to embodimentsof the present invention can also be reduced by reducing the thicknessof a low-index dielectric layer. This reduces the interference color andchroma as well as the color shift in visible wavelengths. As thedielectric layers serve to make the flake mechanically rigid, when thedielectric layers are reduced in thickness, it is may be preferable toincrease the thickness and/or rigidity of the reflective layer or toprovide a stiffening layer.

In another embodiment of the invention, a pigment flake in accordancewith FIG. 2B has a central reflector layer 22 of aluminum about 40 nmthick and a dielectric layer 33 about 1.0 QWOT at 300 nm to about 2.0QWOT at 550 nm in optical communication with the reflector layer. Theabsorber layer 34 has a thickness sufficient to reduce the internaltransmission of the absorber layer to about 50% across the visiblespectrum. The overall average reflectivity of the pigment flake is lessthan about 50% over the visible spectrum.

The visible appearance of the pigment and resultant coatings can beprogressively modified from dark brown to black to indigo to a dull blueto a dull green by adjusting the thickness of the dielectric layer 33.For example, when a dull blue color with low chroma is desired, thedielectric layer thickness is in the range of about 2.0 QWOT @ 280 nm to2.0 QWOT @ 450 nm. Alternatively, a device having a dull green colorwith low chroma results when the dielectric layer thickness increases toa range of about 2 QWOT @ 450 nm to 2 QWOT @ 550 nm. When using alow-index dielectric, increasing the dielectric thickness to greaterthan about 2 QWOT @ 700 nm results in unstable chroma. Unstable chromais a condition where the perceived color depends on the angle ofincident light. For blue and green devices, the reflectance of thedevice reaches a local minimum value in the near infrared, and thenincreases in the mid- and far-infrared regions. By providing generallylow reflectance in the visible and near IR regions, while at the sametime achieving high reflectance in the mid- and far-IR regions, thisembodiment performs as a good solar absorber.

FIGS. 6A and 6B illustrate the calculated performance of anotherembodiment of the present invention. Optical designs according to thisembodiment can incorporated into both foils and pigments, such as athermal control flake with a structure of A/D/R/D/A. The approximatelayer materials and thicknesses for this example device are as follows:

-   -   R=80 nm aluminum    -   D=1.0 QWOT ZnS @ 200 nm    -   A=27 nm carbon.

For this example, the absorber thickness is chosen to provide about 45%internal transmittance (i.e. transmittance in/through the absorberlayer) in the visible region. The design provides a dark (low lightness)burgundy color appearance and a high infrared reflectance. Thedielectric ZnS has a high refractive index, so the reflectance in themid infrared region is generally higher than designs utilizing lowerindex dielectrics, which is an advantage for applications requiringbetter performance in this region. At the same time, the reflectanceminimum in the solar region is not as broad as the design described inassociation with the black optical design. When utilized as a foil, thedesign can be simplified to S/R/D/A, where S is an optional substrate.

FIGS. 7A and 7B are graphical illustrations of the calculated (modeled)optical performance of a multi-layer interference structure according toan embodiment of the present invention over visible wavelengths andinfrared wavelengths. The multi-layer interference structure can beincorporated into both foils and pigments. Rather than utilizing onedielectric-absorber pair, the optical stack has two dielectric-absorberpairs. In a further embodiment, a pigment flake with layer order:A₂/D₂/A₁/D₁/R/D₁/A₁/D₂/A₂ is used. The approximate layer materials andthicknesses for this example device are as follows:

-   -   R=80 nm aluminum    -   D₁=D₂=1.0 QWOT MgF₂ @ 300 nm    -   A₁=7 nm chromium    -   A₂=2.5 nm chromium

The device achieves a uniform low reflectance across the visiblespectrum but achieves a high reflectance level (low emittance) in thefar infrared. Instead of two dielectric-absorber pairs, three and fourpairs can be used; however, this increases cost and complexity. Whenutilized as a foil, the design can be simplified to S/R/D₁/A₁/D₂/A₂where S is an optional substrate. In some embodiments, the reflector issubstantially thicker than necessary to provide the desired reflectance,and serves as a substrate to deposit the other layers on, such asaluminum foil or stainless steel foil.

V. Forms and Applications of Embodiments of the Invention

Optical thin film structures according to embodiments can be formed onpigment particles, films, foils, and other objects. For example, aoptical thin film structure might be deposited directly on a piece ofglass, metal, plastic, ceramic, or composite material, or on a film orfoil that is applied to a piece of wood, fabric, plastic, glass, metal,ceramic, or composite material.

FIG. 8A is a simplified cross section of a portion of a film or foil 90according to an embodiment of the present invention. An optical thinfilm structure 91 according to an embodiment of the present invention isdeposited on a substrate 89, such as a polymer film, e.g. a film ofpolyethylene terephthalate (“PET”) or a metal foil, such as aluminum orstainless steel foil. An optional top coating 92 is applied over theoptical thin film structure to provide protection from the environment,or to enhance the optical properties of the film or foil. In analternative embodiment, a polymer cover sheet is attached to the top ofthe optical thin film structure with a layer of laminating adhesive. Anoptional mounting adhesive layer 93 is provided on the opposite side ofthe substrate, and a release liner 94 is provided to facilitate handlingof the assembly. The release liner is removed to expose the mountingadhesive layer so the film or foil can be conveniently applied to asurface.

FIG. 8B is a simplified cross section of a foil 95 as-described inaccordance with FIG. 8A, above, attached to a panel 96. The releaseliner is removed to expose the mounting adhesive, and the film isapplied to the panel. Films according to embodiments of the presentinvention provide thermal control by having low IR emittance (high IRreflectivity) while transmitting or absorbing higher wavelength light.

FIG. 8C is a simplified cross section of an optical thin film transferfoil 111 for application to a surface. The optical thin film stack 91′may be deposited “backwards” onto the substrate 89′, such as a plasticfilm. In other words, the absorber is deposited first, then the spacerlayer, and finally the reflector. An optional adhesive layer 93′ andrelease liner 94 may be included for attaching the transfer foil to asurface, or the surface may be coated with the adhesive before attachingthe foil. An optional release layer 110 may be included between thesubstrate and the thin film layer to facilitate removal of the substratefrom the transfer foil after it has been attached to the surface. Anoptional overcoat layer (not shown) may be applied between the opticalthin film stack and the release layer before applying the transfer foilto the surface, or may be applied after the transfer foil has beenattached to the surface.

Pigment flakes according to embodiments of the present invention can beincorporated into any number of liquid or solid media and used as ink,paint, extruded plastic film, plastic part, or powder coatings, forexample. The optical designs have advantages where a low chroma and/orrelatively neutral color effect is desired along with low infraredemittance. The dielectric and absorber thicknesses of the design arechosen to meet the desired color, lightness, reflectance, solarabsorptance, and infrared emittance properties. Embodiments of thepresent invention allow one to design a foil, pigment, or otherstructure incorporating an optical stack to give a wide range of solarabsorptance, infrared emittance, lightness, and color values. Designswith thin absorbers, i.e. with absorber layers that are thinner than theabsorber thickness that would provide the minimum reflectance in thevisible spectrum, will have higher mid-infrared reflectance than willdesigns with thicker absorbers. The optical design of the pigment flakesmay be optimized according to the type of carrier or vehicle (“matrix”)the pigment flakes will be dispersed in. In some instances, the matrixhas a relatively low refractive index, and optical designs derived forair can be used in the carrier with little degradation of the opticalperformance of the pigment dispersed in the matrix.

FIG. 9A is a simplified cross section of a polymeric sheet 97 withpigment flakes 98 according to an embodiment of the present invention.Such a polymeric sheet can be formed by casting or by extrusion. Thefilm forming the sheet should have a thickness of at least two times thethickness of the multi-layer interference thin film flakes, whichtypically have a thickness of approximately 0.5 micrometer, so that thesheet should have a thickness of about 1 μm or more.

In general, the polymeric sheet comprises a layer of polymer materialhaving first and second parallel surfaces 99, 100. A plurality ofpigments flakes according to an embodiment of the present invention aredisposed within the layer of polymeric material. The flakes generallyhave an aspect ratio not less than about 2:1 and first and secondparallel surfaces that generally align themselves to the first andsecond parallel surfaces of the polymeric sheet so that the aggregateeffect of the particles is similar to the effect that might be obtainedby a foil or film as described above in relation to FIG. 8A. Alignmentof the flakes can be achieved during extrusion or casting, or bystretching the polymer sheet, for example. An adhesive layer and releaseliner may be further added to one or both sides of the polymeric sheet.Similarly, the sheet may be laminated to additional polymeric sheets orother objects. Alternatively, a multiple layer polymeric sheet can beformed by co-extruding films in which the outermost, and preferablythinner layer, comprises the pigments of the instant invention.

FIG. 9B is a simplified cross section of a co-extruded polymer sheet 101with pigment flakes according to another embodiment of the presentinvention. The a first film layer 102 in such a co-extruded film mayprovide structural support or protection to the film layer 103containing the pigment flakes, serve as an adhesion or bonding layer tothe article or housing requiring thermal control or serve as thestructural housing itself. In other embodiments, a central film layermay be extruded with pigment-containing layers on either side, or apigment containing layer may be sandwiched between two film layers, suchas clear or dyed PET.

In connection with the present invention, various types of polymers canbe used. For example, with an aqueous polymer, a polyvinyl alcohol,polyvinyl acetate polyvinylpyrrolidone, poly(ethoxyethylene),poly(methoxyethylene), poly(acrylic) acid, poly(acrylamide),poly(oxyethylene), poly(maleic anhydride), hydroxyethyl cellulose,cellulose acetate and poly(sacchrides) such as gum arabic and pectin maybe used. If an organic solvent base is to be utilized, almost anypolymer system that is dissolvable may be used. This may include thosepolymers listed in the aqueous examples above but will also include theadditional polymers of poly(acetals), such as polyvinylbutyral,poly(vinyl halides), such as polyvinyl chloride and polyvinylenechloride, poly(dienes) such as polybutadiene, poly(alkenes) such aspolyethylene, poly(acrylates) such as polymethyl acrylate,poly(methacrylates) such as poly methylmethacrylate, poly(carbonates)such as poly(oxycarbonyl oxyhexamethylene, poly(esters) such aspolyethylene terephthalate, poly(urethanes), poly(siloxanes),poly(suphides), poly(sulphones), poly(vinylnitriles),poly(acrylonitriles), poly(styrene), poly(phenylenes) such as poly(2,5dihydroxy-1,4-phenyleneethylene), poly(amides), natural rubbers,formaldahyde resins and other polymers.

FIG. 10 is a simplified cross section of a layer of paint 104 containingpigment flakes 98 according to an embodiment of the present inventionapplied to a surface 105. The paint includes a paint vehicle 106 that isfluid when the paint is applied. As the paint dries or hardens, thepigment flakes generally align themselves with the surface. Thus, theaggregate effect of the pigment particles is similar to the surfacebeing covered with a foil or film, or being directly coated with thethin film layers having a similar optical thin film structure. In aparticular embodiment, the paint matrix is a polymer with a low infraredemittance, such as a silicon-based paint. In a particular embodiment, itis desirable that the vehicle or matrix of the pigment flakes has an IRemittance less than about 0.5.

The current invention offers the advantages listed in the previoussection in that one can provide a portable thin film applicable as apaint, ink, plastic, or other form that has substantially lowerreflectance values in the visible portion of the optical spectrumcompared to the infrared portion of the spectrum. Thus, one can providea variety of brightness and colors with low infrared emittance. Thereflectance characteristics can be further selected between the near-and far-infrared regions. Paints according to embodiments of theinvention may be indigo, blue, green, brown, and burgundy, as well asblack and various shades of gray.

In the energy control field, this invention allows for solar controlpaints with high absorptance and low emittance that can be applied to avariety of surfaces and materials. In addition, it allows thermalcontrol paints with specified absorptance/emittance ratios. Pigmentsmade according to the teachings of the invention to have a dark or blackappearance are useful in decorative, military, or solar energy markets.

Pigments made according to embodiments of the present invention are alsouseful in anti-counterfeiting applications. Pigments having a neutral ornear neutral appearance (low chroma or very low chroma) and high IRreflectivity can be used to impart an IR image to an object. In furtherembodiments, neutral or low chroma ink, paint, foil, or similar materialhaving high IR reflectivity is patterned in conjunction with similarneutral or low chroma with lower IR reflectivity. For example, an imagecould be printed on a bill, certificate, passport, or other article thathas one image visible to the eye, and another image visible in theinfrared, such as overprinting an IR image with black pigment accordingto the present invention in a black field printed with conventionalblack ink.

FIG. 11 is a simplified plan view of an image 107 printed with inkaccording to an embodiment of the present invention. The image has afirst field 108 and a second field 109. For simplicity of illustrations,both fields are shown without color or shading, but would generally beblack, gray, or low-color, at least for one field of the image. Forexample, the first field could be printed with conventional black ink,and the second field printed with black ink according to an embodimentof the present invention. The second field could have relatively high IRreflectivity and be easily viewed with an IR detector, but essentiallybe invisible to a human. Such techniques may be used to superimpose IRimages on visual images, or to embed IR images on or in objects, such asfor security or anti-counterfeiting purposes. Similarly, the entireimage could be printed with ink according to the present invention,which would not be obvious by casual inspection, yet be verifiable withIR inspection techniques, and would be much more involved for thecounterfeiter to make.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges that come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

1-73. (canceled)
 74. An optical structure comprising: a reflector; aspacer layer with a spacer thickness disposed on the reflector; and anabsorber layer disposed on the spacer layer, the absorber layer havingan absorber thickness providing a transmittance through the absorberlayer between 5-85% wherein the reflector has a reflector thickness, thespacer thickness and the absorber thickness are selected to achieve anaverage first reflectivity of not more than 50% between 400-700nanometers and an average second reflectivity of not less than 50%between 4-40 microns.
 75. The optical structure of claim 74 wherein thespacer thickness is between 1 quarter-wave optical thickness at 200 nmand 2 quarter-wave optical thicknesses at 700 nm.
 76. The opticalstructure of claim 74 wherein the spacer thickness is selected toachieve a reflectivity minimum of the optical interference structurebetween 200-1500 nm.
 77. The optical structure of claim 74 wherein thespacer thickness is less than one quarter-wave optical thickness at 700nm and is selected to achieve a reflectivity minimum of the opticalstructure between 200-700 nm.
 78. The optical structure of claim 74wherein the reflector comprises aluminum, the spacer layer comprisesmagnesium fluoride and the absorber layer comprises chromium having aninternal transmittance of not more than 50%.
 79. The optical structureof claim 74 wherein the spacer layer is made of a selectively absorbingspacer material being more absorptive in the visible range than in theinfrared range.
 80. The optical structure of claim 79 wherein theselectively absorbing spacer material is selected from the groupconsisting of iron oxide, tungsten oxide, copper oxide, and cobaltoxide.
 81. The optical structure of claim 74 wherein the spacer layercomprises a spacer material selected from the group consisting of zincsulfide, zinc oxide, zirconium oxide, titanium dioxide, diamond-likecarbon, indium oxide, indium-tin-oxide, tantalum pentoxide), cericoxide, yttrium oxide, europium oxide, iron oxide, ferric oxide, hafniumnitride, hafnium carbide, hafnium oxide, lanthanum oxide, magnesiumoxide, neodymium oxide, praseodymium oxide, samarium oxide, antimonytrioxide, silicon, silicon monoxide, germanium, selenium trioxide, tinoxide, tungsten trioxide, and combinations thereof.
 82. The opticalstructure of claim 74 wherein the spacer layer comprises a spacermaterial selected from the group consisting of silicon dioxide; aluminumoxide; metal fluoride, including magnesium fluoride, aluminum fluoride,cerium fluoride, lanthanum fluoride, sodium aluminum fluorides includingNa₃AlF₆ and Na₅Al₃F₁₄, neodymium fluoride, samarium fluoride, bariumfluoride, calcium fluoride, and lithium fluoride; and organic monomersand polymers, including dienes, alkenes, acrylates includingmethacrylate, perfluoroalkenes, polytetrafluoroethylene, and fluorinatedethylene propylene (“FEP”).
 83. The optical structure of claim 74wherein the absorber thickness is selected to achieve a reflectivityminimum of less than 10% reflectivity of the optical interferencestructure between 200-1500 nm.
 84. The optical structure of claim 74wherein the absorber thickness is selected to achieve a reflectivityminimum of the optical interference structure between 200-1500 nm. 85.The optical structure of claim 74 disposed on a plurality of pigmentflakes in a paint vehicle to provide a paint formulation.
 86. Theoptical structure of claim 85 wherein the paint vehicle has a lowinfrared emittance.
 87. The optical structure of claim 85 disposed on asurface to provide a solar absorber.
 88. The optical structure of claim85 disposed on a surface to provide infrared camouflage.
 89. The opticalstructure of claim 74 disposed on a plurality of pigment flakes in anink vehicle to provide an ink.
 90. The optical structure of claim 74disposed on a surface to provide an infrared image.
 91. The opticalstructure of claim 90 wherein the infrared image is not discernible byan unaided human eye.
 92. The optical structure of claim 90 wherein theinfrared image is incorporated in a heat-sensitive foil pressed onto asurface.
 93. The optical structure of claim 74 wherein the opticalstructure has indiscernible color shift when viewed by a human eyethrough a viewing arc of 0-90 degrees measured from a normal to a majorsurface of the optical structure with a fixed illumination source. 94.The optical structure of claim 74 disposed on a plurality of pigmentflakes in a polymeric sheet.
 95. The optical structure of claim 94wherein the polymeric sheet comprises a material selected from the groupconsisting of aqueous polymer, polyvinyl alcohol, polyvinyl acetatepolyvinylpyrrolidone, poly(ethoxyethylene), poly(methoxyethylene),poly(acrylic) acid, poly(acrylamide), poly(oxyethylene), poly(maleicanhydride), hydroxyethyl cellulose, cellulose acetate andpoly(sacchrides) including gum arabic and pectin, poly(acetals)including polyvinylbutyral, poly(vinyl halides) including polyvinylchloride and polyvinylene chloride, poly(dienes) includingpolybutadiene, poly(alkenes) including polyethylene, poly(acrylates)including polymethyl acrylate, poly(methacrylates) including polymethylmethacrylate, poly(carbonates) including poly(oxycarbonyloxyhexamethylene, poly(esters) including polyethylene terephthalate,poly(urethanes), poly(siloxanes), poly(suphides), poly(sulphones),poly(vinylnitriles), poly(acrylonitriles), poly(styrene),poly(phenylenes) including poly(2,5 dihydroxy-1,4-phenyleneethylene),poly(amides), natural rubbers, formaldahyde resins, and combinationsthereof.
 96. The optical structure of claim 94 wherein the polymericsheet is an extruded sheet.
 97. The optical structure of claim 94wherein the polymeric sheet is stretched.
 98. The optical structure ofclaim 74 disposed on a plurality of pigment flakes and mixed with apowder coating vehicle to provide a powder coating formulation.
 99. Theoptical structure of claim 98 wherein the powder coating vehicle isclear at visible wavelengths.
 100. The optical structure of claim 74disposed on a film.
 101. The optical structure of claim 100 wherein thefilm includes an adhesive layer.
 102. The optical structure of claim 74disposed on a foil.
 103. The optical structure of claim 102 wherein thefoil comprises aluminum or stainless steel.
 104. The optical structureof claim 74 wherein the absorber thickness is less than 3 timestabs_(min), the minimum visible reflectance absorber thickness providinga minimum reflectivity of the optical interference structure between200-1500 nm.
 105. The optical structure of claim 74 wherein the absorberlayer comprises an absorber material selected from the group consistingof chromium, nickel, iron, titanium, aluminum, tungsten, molybdenum,niobium, metal alloys, including Ni—Cr—Fe alloy, metal dispersed in adielectric matrix, iron oxide (Fe₂O₃), silicon monoxide (SiO), chromiumoxide (Cr₂O₃), carbon, titanium nitride (TiN), and titanium sub-oxide(TiO_(x) where x is less than 2.0).
 106. The optical structure of claim74 wherein the reflector, spacer layer, and absorber layer are chosen soas to achieve a chroma of less than 20 for the optical interferencestructure.
 107. The optical structure of claim 74 wherein the reflectorlayer comprises a reflector material selected from the group consistingof aluminum, silver, iron, tantalum, iridium, rhenium, copper, silver,gold, platinum, palladium, nickel, cobalt, niobium, chromium, tin,alloys, metal carbides, metal oxides, metal nitrides, metal sulfides.108. The optical structure of claim 74 wherein the reflector layercomprises a reflector material selected from the group consisting ofindium oxide, indium tin oxide (ITO), europium oxide (Eu₂O₃), vanadiumpentoxide (V₂O₅), rhenium oxide (ReO₃), lanthanum boride (LaB₆). 109.The optical structure of claim 74 wherein the spacer layer has a spacerlayer thickness of 1 quarter-wave optical thickness for wavelengthsbetween 100-500 nm and further comprising a second spacer layer disposedon the absorber layer, the second spacer layer having a second spacerlayer thickness of 1 quarter-wave optical thickness for wavelengthsbetween 100-500 nm; and a second absorber layer having an internaltransmittance between 5% and 85% disposed on the second spacer layer.110. The optical structure of claim 74 wherein the spacer thickness isone quarter-wave optical thickness between 200-800 nm, the reflectorcomprises aluminum reflector and the absorber layer comprises 20 nm ofchromium.
 111. The optical structure of claim 74 wherein the opticalstructure is formed on a flake substrate.
 112. The optical structure ofclaim 111 wherein at least one of the reflector, the spacer layer, andthe absorber layer surrounds the flake substrate.
 113. The opticalstructure of claim 74 wherein the optical structure has an opticalstructure thickness not greater than 250 nm and is formed on a stiffflake substrate.
 114. The optical structure of claim 74 furthercomprising a stiffening layer.
 115. The optical structure of claim 74further comprising an overcoat layer having an overcoat layer thickness.116. The optical structure of claim 115 wherein the overcoat layerthickness is at least eight times the spacer layer thickness.
 117. Theoptical structure of claim 115 wherein the overcoat layer thickness isless than eight times the spacer layer thickness.
 118. The opticalstructure of claim 74 having a chroma less than
 5. 119. The opticalstructure of claim 74 having an average reflectance not greater than 20%between 400-700 nm.
 120. An optical structure comprising: a reflector; aspacer layer with a spacer thickness disposed on the reflector; and anabsorber layer disposed on the spacer layer, the absorber layer havingan absorber thickness providing a transmittance through the absorberlayer between 5-85%, the spacer thickness being selected so as toprovide an average reflectance of the optical interference structureless than 50% reflectivity between 400-700 nm and a chroma of theoptical interference structure less than
 20. 121. The optical structureof claim 120 wherein the spacer thickness is between one quarter-waveoptical thickness at 300 nm and two quarter-wave optical thicknesses at550 nm.
 122. The optical structure of claim 120 wherein the spacerthickness is between 2 quarter-wave optical thicknesses at a firstwavelength of 280 nm and 2 quarter-wave optical thicknesses at a secondwavelength of 450 nm.
 123. The optical structure of claim 120 whereinthe spacer thickness is between 2 quarter-wave optical thickness at afirst wavelength of 450 nm and 2 quarter-wave optical thicknesses at asecond wavelength of 550 nm.
 124. An optical interference structurecomprising: a reflector having a reflectivity of at least 50% over awavelength range of 4-40 microns; a spacer layer disposed on thereflector having a spacer layer thickness of between 1 quarter-waveoptical thickness at a first wavelength of 200 nm and 2 quarter-waveoptical thicknesses at a second wavelength of 500 nm, the spacer layerhaving a refractive index less than 2; and an absorber layer with anabsorber layer thickness selected to provide an average reflectance ofthe optical interference structure less than 20% between 400-700 nm.125. The optical interference structure of claim 124 wherein the opticalinterference structure has a chroma less than
 20. 126. The opticalinterference structure of claim 124 further comprising: a second spacerlayer with a second spacer thickness disposed on the absorber layer; anda second absorber layer disposed on the second spacer layer.
 127. Theoptical interference structure of claim 126 wherein the spacer layerthickness is less than one quarter-wave optical thickness at awavelength of 700 nm and the second spacer layer thickness is less thanone quarter-wave optical thickness at the wavelength of 700 nm.
 128. Theoptical interference structure of claim 124 wherein the substrate is analuminum flake comprising the aluminum reflector and the second aluminumreflector.
 129. The optical interference structure of claim 124 whereinthe substrate is a substantially rigid dielectric flake.
 130. Theoptical interference structure of claim 129 wherein the substantiallyrigid dielectric flake comprises glass, mica, alumina, iron oxide,graphite, bismuth oxychloride, boron nitride, or polymer.
 131. Anoptical interference structure comprising: an aluminum reflector; afirst dielectric layer disposed on the aluminum reflector, the firstdielectric layer comprising MgF₂ having a first dielectric thickness of55 nm; a first absorber layer comprising 7 nm of chromium disposed onthe first dielectric layer; a second dielectric layer disposed on thefirst absorber layer, the second dielectric layer comprising MgF₂ havinga second dielectric thickness of 55 nm; and a second absorber layercomprising 2.5 nm of chromium disposed on the second dielectric layer,the second absorber layer.
 132. The optical interference structure ofclaim 131 wherein the aluminum reflector is an opaque reflector layernot less than 30 nm thick.
 133. The optical interference structure ofclaim 131 further comprising: a substrate having a first surface and asecond surface, the aluminum reflector being disposed on the firstsurface of the substrate; a second aluminum reflector disposed on thesecond surface of the substrate; a third dielectric layer disposed onthe second aluminum reflector, the third dielectric layer comprisingMgF₂ having a third dielectric thickness of 55 nm; a third absorberlayer disposed on the third dielectric layer comprising 7 nm ofchromium; a fourth dielectric layer disposed on the third absorberlayer, the fourth dielectric layer comprising MgF₂ having a fourthdielectric thickness of 55 nm; and a fourth absorber layer disposed onthe fourth dielectric layer, the second absorber layer comprising 2.5 nmof chromium.
 134. An optical interference structure comprising: asubstantially opaque aluminum reflector a dielectric layer comprisingZnS being 1 quarter-wavelength optical thickness at a wavelength between200-1400 nm; and an absorber layer having an internal transmittancebetween 5-85%.
 135. An optical interference structure comprising: analuminum reflector a dielectric layer comprising MgF₂ being 1quarter-wavelength optical thickness at a wavelength of 500 nm; and anabsorber layer comprising 20 nm of chromium.
 136. The opticalinterference structure of claim 135 having a chroma less than 5.